In order to improve the chemical and mechanical properties of a substrate, e.g., hardness, wear resistance, corrosion resistance, and fatigue resistance, the substrate, such as one used for a razor blade, have hard coatings applied to the substrate, particularly at a cutting edge. For razor blades, these hard coatings, which are typically applied on the cutting edge, serve two major roles, namely to strengthen the razor blade, which allows for slimmer profiles, and to provide a suitable interface for the adhesion of a telomer coating. Generally, the thinner the razor blade becomes at its cutting edge, the lower the cutting force and the better the razor blade's cutting attributes. Examples of cutting edge structures comprising lower cutting forces are described in U.S. Pat. Nos. 5,295,305; 5,232,568; 4,933,058; 5,032,243; 5,497,550; 5,940,975; 5,669,144; 5,217,010; and 5,142,785. The properties of the resulting razor blade greatly depend on the strength and the hardness of both the underlying substrate and the coating.
Deflection or bending of the razor blade is a major factor in providing a close and comfortable shave to a user. Some of the discomfort during and after shaving, usually due to nicking, results from excessive stiffness of the razor blade because of the coatings, e.g., amorphous diamond, that are present. This phenomenon is due to the hard coatings actually changing the profile of the razor blade's cutting edge because the thickness is typically higher towards the tip.
Typically, the hard coatings on a razor blade's cutting edge have a multilayered structure. These layers are usually selected and optimized to provide sufficient strength and adhesion of a fluoropolymer-containing, e.g., polytetrafluoroethylene (PTFE), coating. However, these coatings are not optimized to provide appropriate strength and flexibility to the underlying substrate itself. While the hard coating may contribute to the overall strength of the razor blade, neither of these coatings contribute to minimizing the coating thickness.
Instead of adding a hard coating, there have been attempts at hardening the substrate by nitriding, which is a technique used to strengthen ferrous and non-ferrous materials. When applied to steel substrates, such as those used for razor blades, compound layers of various Fe—N phases, namely the cubic gamma prime (Fe4N) phase or the hexagonal epsilon (Fe2-3N) phase, are produced. However, these gamma prime and epsilon phases do not actually increase the strength of the underlying substrate, but instead produce a brittle substrate surface, particularly at the cutting edge, because of byproduct growth or hillocks that are formed. If the cutting edge breaks on a razor blade, the remaining razor blade will yield an extremely uncomfortable shave.
One of the major advantages of plasma nitriding over gas and salt-bath nitriding is the process control. By controlling the power and the gas composition, the phase composition, layer structure and its thickness as well as other properties of treated stainless steel substrates can be controlled. For example, nitriding stainless steel increases the abrasion resistance of the underlying steel as well as improves the fatigue strength and reduces the friction coefficient.
Accordingly, it would be advantageous to produce an improved razor blade, which does not include the gamma prime or epsilon phases. Instead, the improved razor blade includes a mixed nitride-substrate interregion that contains a solid solution of nitrogen, which does not materially alter the original razor blade profile. This configuration provides improved razor blade properties, such as a slimmer cutting edge. Furthermore, it would also be advantageous to improve the shaving performance by lowering the cutting force of the razor blade's cutting edge. This could be achieved by applying a significantly thinner hard coating, or possibly eliminating it by strengthening the substrate prior to applying the hard coating.